(Keynote) Novel Materials for Catalytic Cascades: Integrating Experimental and Computational Strategies

Abstract

Many groups have begun to explore spatial organization of enzymatic active sites that can lead to enhanced pathways in synthetic multi-step reactions such as electro-oxidation of complex oxygenated fuels. However, it was shown that any further improvements could be obtained only by integration of new catalytic modalities, such as abiotic catalysts, into the framework of multi-enzyme cascades. Herein we report how computational efforts can be combined with experimental efforts to develop novel catalytic materials and assist their integration in more complex catalytic systems. We first report the synthesis of hybrid Pd/Mn-N-3D-Graphene nanosheets (Pd/Mn-N-3D-GNS) catalyst for oxidation of oxygenated molecules that are key intermediates in the multistep oxidation of various organic compounds. Synthetized material was further electrochemically evaluated towards the oxidation of D-fructose, glycerol, glyceraldehyde, hydroxyacetone, hydroxypyruvate, glyoxalic acid, mesoxalic acid, oxalic acid, and formic acid. Based on the generated currents it was found that Pd/Mn-N-3D-GNS demonstrates the highest activity toward oxalic acid, followed by mesoxalic and glyoxalic acid. Comparison in the activity among catalytic components, namely Pd/Mn-N-3D-GNS, Pd/3D-GNS, and Mn-N-3D-GNS, revealed that the observed currents are due to the presence of Mn-N-C catalytic active sites, while the decrease in the onset potential can be attributed to Pd. It was further found that Mn containing catalysts do not perform well as electro-catalysts for the oxidation of formic acid. Cyclic voltammograms measured at pH=5.2 showed that Mn-N-3D-GNS and Pd/Mn-N-3D-GNS oxidize formic acid via unfavorable indirect route with the production of CO, which leads to the poisoning and deactivation of the catalysts. Pd/3D-GNS, on the other hand, is capable of oxidizing formic acid via a direct route with minimal production of CO. We further aim at creating assemblies of the abiotic catalysts based on Pd and Pt together with oxalate dehydrogenase to catalyze final key steps in the fructose oxidation cascade. Oxalate dehydrogenase can effectively convert mesoxalic acid to formic acid, however a different catalyst is required to convert formic acid to CO2. It is expected that the incorporation of the abiotic catalysts with oxalate dehydrogenase in an optimal way will lead to increased efficiency of the overall catalytic process. In the second part of the talk we will demonstrate how ab initio methods and docking simulations can be combined in order to understand and improve incorporation of abiotic and biotic catalysts in the framework of multi-step fructose oxidation. The interaction of oxalic, mesoxalic, and glyoxalic acid with the Mn-containing catalysts and Pt-based catalysts was first studied using density functional theory (DFT). The results reveal the role of alloying component in the oxidation of simple oxygenated molecules by Pt-based catalysts. Correlation between adsorption energies of the reactants and the onset-potentials and adsorption energies and generated currents further suggest that the interaction of reactants with the Mn-N-C sites and metal surfaces affects the kinetically determining step. Finally, we use an example of bilirubin oxidase to illustrate how DFT and docking simulations can be combined to study support-substrate-enzyme interface crucial for the efficient incorporation of bio-catalysts in the synthetic catalytic cascades. Figure 1